Cause it would make Object Desktop cost more. Object Desktop (in my opinion) includes programs that basically work together with each other but ObjectDock and CursorFX tend to stand alone even with a theme suit from my own experience. Besides, they alone can be apealing to many who are not really interested in changing the look of everyhing on their screen. Animated cursors can be easier to spot on the screen and a dock with all your shortcuts can be very handy even without Windowblinds.

From a thread I saw several years ago the explanation was more about the contract with the individual program developers

My car came with gas, a full tank as a matter of fact. What you may have meant to say When you buy a car, why isn't the insurance included?